[Zeke]

The U-channel is for putty. For installation you move the gland nuts forward to expose just enough thread for the finish nut/washer/putty assembly. On the back there should be a brass flat washer between the bgand nut and the back of the sink. Also there would be a fiber washer directly against the cast iron back since it's not smooth.

Rubber o-rings work too, but they don't last forever and can stain. You need to buy good ones from a plumbing supply that won't dry out and crack. I see a lot of red ones on old work.

The idea of using silicone grease is good with rubber because it can bunch up while cinching down.